User equipment using hybrid automatic repeat request

ABSTRACT

A user equipment comprises a transmitter and an adaptive modulation and coding controller. The transmitter is configured to transmit data over an air interface in a single transmission time interval with a first specified modulation and coding scheme, where the single transmission time interval has a plurality of transport block sets. In response to receiving a repeat request for retransmission of at least one particular transport block set, the transmitter retransmits the at least one of the particular transport block sets. The adaptive modulation and coding controller is configured to change the specified modulation and coding scheme to a second specified modulation and coding scheme, enabling a combining of a particular transport block set transmitted at the first specified modulation and coding scheme with a retransmitted version of the particular transport block set transmitted at the second specified modulation and coding scheme.

This application is a continuation of U.S. patent application Ser. No.10/279,393, filed Oct. 24, 2002, which claims priority to U.S.Provisional Application No. 60/357,224, filed Feb. 13, 2002, both ofwhich are incorporated herein by reference as if fully set forth.

BACKGROUND

This invention generally relates to wireless communication systems. Inparticular, the invention relates to transmission of data in suchsystems where adaptive modulation & coding (AMC) and hybrid automaticrepeat request (H-ARQ) techniques are applied.

In wireless communication systems, such as the third generationpartnership project (3GPP) time division duplex (TDD) or frequencydivision duplex (FDD) communication systems using code division multipleaccess (CDMA) or orthogonal frequency division multiplex (OFDM) systems,AMC is used to optimize the use of air resources.

The modulation and coding schemes (sets) used to transmit data arevaried based on wireless channel conditions. To illustrate, a type oferror encoding (such as turbo versus convolutional coding), coding rate,spreading factor for CDMA system, modulation type (such as quadraturephase shift keying versus M-ary quadrature amplitude modulation), and/oradding/subtracting sub-carriers for an OFDM system may change. Ifchannel characteristics improve, a lower data redundancy and/or “lessrobust” modulation and coding set is used to transfer data. As a result,for a given allocation of radio resources, more user data is transferredresulting in a higher effective data rate. Conversely, if channelcharacteristics degrade, a higher data redundancy “more robust”modulation and coding set is used, transferring less user data. UsingAMC, an optimization between air resource utilization and quality ofservice (QOS) can be better maintained.

Data in such systems is received for transfer over the air interface intransmission time intervals (TTIs). Data within a TTI transferred to aparticular user equipment is referred to as a transport block set (TBS).For a particular allocation of air resources, a less robust modulationand coding set allows for larger TBS sizes and a more robust modulationand coding set only allows for smaller TBS sizes. As a result, themodulation and coding set for a given radio resource allocation dictatesthe maximum size of the TBS that can be supported in a given TTI.

In such systems, a hybrid automatic repeat request (H-ARQ) mechanism maybe used to maintain QOS and improve radio resource efficiency. A systemusing H-ARQ is shown in FIG. 1. A transmitter 20 transmits a TBS overthe air interface using a particular modulation and coding set. The TBSis received by a receiver 26. An H-ARQ decoder 30 decodes the receivedTBS. If the quality of the received data is unacceptable, an ARQtransmitter 28 requests a retransmission of the TBS. One approach tocheck the quality of the received TBS is a cyclic redundancy check(CRC). An ARQ receiver 22 receives the request and a retransmission ofthe TBS is made by the transmitter 20. Retransmissions may apply a morerobust modulation and coding set to increase the possibility ofsuccessful delivery. The H-ARQ decoder 30 combines, the received TBSversions. A requirement for combining is that combined TBSs areidentical. If the resulting quality is still insufficient, anotherretransmission is requested. If the resulting quality is sufficient,such as the combined TBS passes the CRC check, the received TBS isreleased for further processing. The H-ARQ mechanism allows for datareceived with unacceptable quality to be retransmitted to maintain thedesired QOS.

In a system using both H-ARQ and AMC, a change in modulation and codingset may be determined necessary to achieve successful delivery of arequested TBS retransmission. In this situation, the maximum amount ofphysical data bits allowed within the TTI varies with the modulation andcoding set.

Since only one TBS exists per TTI the effective user data ratecorresponds to the TBS size applied to each TTI. To achieve maximum datarates the largest TBS size is applied to the least robust modulation andcoding set within the TTI. When wireless channel conditions require amore robust modulation and coding set for successful transmission, suchas when a TBS size can not be supported within the TTI. Therefore, whenoperating at the maximum data rate, each time a more robust modulationand coding requirement is realized, all outstanding transmissions inH-ARQ processes that have not been successfully acknowledged must bediscarded.

When Incremental Redundancy (IR) is applied, TBS data must remainconstant in retransmissions for proper combining. Therefore, toguarantee that a TBS retransmission can be supported at a more robustmodulation and coding set then the initial transmission, the TBS sizeused must correspond to the most robust MCS. However, when a TBS sizeallowed by the most robust modulation and coding set is applied themaximum data rate to the mobile is reduced, and when a less robustmodulation and coding set is applied physical resources are not fullyutilized.

When the TBS size is not supported by the more robust modulation andcoding set, the TBS can be retransmitted using the old modulation andcoding set. However, if the channel conditions dictate that a morerobust modulation and coding set be used or the initial transmission wasseverally corrupted, the combining of the retransmitted TBSs may neverpass, resulting in a transmission failure.

In current implementations, when a TBS can not be successfullytransmitted by AMC & H-ARQ mechanisms, recovery is handled by the radiolink control (RLC) protocol (at layer two). Unlike a H-ARQ recovery offailed transmissions, the RLC error detection, data recovery andbuffering of a TBS queued in the node-B, results in increased blockerror rates and data latency, potentially resulting in a failure to meetQOS requirements.

Accordingly, to provide maximum data rates with minimal H-ARQtransmission failures, it is desirable to support incremental redundancyand allow adaptation of modulation and coding sets in such systems.

SUMMARY

A user equipment comprises a transmitter and an adaptive modulation andcoding controller. The transmitter is configured to transmit data overan air interface in a single transmission time interval with a firstspecified modulation and coding scheme, where the single transmissiontime interval has a plurality of transport block sets. In response toreceiving a repeat request for retransmission of at least one particulartransport block set, the transmitter retransmits the at least one of theparticular transport block sets. The adaptive modulation and codingcontroller is configured to change the specified modulation and codingscheme to a second specified modulation and coding scheme, enabling acombining of a particular transport block set transmitted at the firstspecified modulation and coding scheme with a retransmitted version ofthe particular transport block set transmitted at the second specifiedmodulation and coding scheme.

BRIEF DESCRIPTION OF THE DRAWING(S)

FIG. 1 is an embodiment of a wireless H-ARQ communication system.

FIGS. 2A-2D are illustrations of a TTI having multiple TBSs.

FIGS. 3A-3C are embodiments of a wireless H-ARQ communication systemusing AMC with TTIs capable of having multiple TBSs.

FIG. 4 is a flow chart of changing the modulation and coding set priorto a H-ARQ retransmission.

FIG. 5 is an illustration of changing the modulation and coding setprior to a retransmission of a single TBS.

FIG. 6 is an illustration of changing the modulation and coding setprior to a retransmission of all three TBSs.

FIG. 7 is an illustration of overlapping TBSs in a TDD/CDMAcommunication system.

FIG. 8 is an illustration of non-overlapping TBSs in a TDD/CDMAcommunication system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

FIGS. 2A, 2B, 2C and 2D illustrate a TTI having multiple TBSs, TBS₁ toTBS_(N). FIG. 2A illustrates multiple TBSs dividing a TTI by time, suchas for use in a TDD/CDMA system. FIG. 2B illustrates multiple TBSsdivided by codes, such as for use in a FDD/CDMA or TDD/CDMA system. FIG.2C illustrates dividing multiple TBSs by time and codes, such as for usein TDD/CDMA system. FIG. 2D illustrates dividing multiple TBSs bysub-carriers, such as for use in an OFDM system. Each TBS is sized toallow transmission with the most robust modulation coding set for theallocated resources. To illustrate, the most robust MCS may only havethe capacity to support a maximum 2,000 bit TBS within the TTI. Althoughreferred to as the most robust modulation coding set, in practice, themost robust set may actually be a more robust set, if the most robustmodulation coding set is unlikely to be needed. The least robustmodulation and coding set may have the capacity to support a maximum of20,000 bit TBS within the TTI. Although referred to as the least robustmodulation coding set, in practice, the least robust set may actually bea less robust set, if the least robust modulation coding set is unlikelyto be needed.

The TBS is sized, preferably, to allow for transmission with the mostrobust modulation and coding set within a TTI. Then when the leastrobust modulation and coding set is applied, multiple TBSs of this sizeare applied within the TTI to achieve maximum data rates, and whengreater transmission reliability is required for successful delivery themost robust modulation and coding set can be applied.

FIG. 3A is a simplified diagram of a transmitter 44 and receiver 46 fortransmitting a TTI having one or multiple TBSs. The transmitter 44 maybe located at either a user equipment or a base station/Node-B. Thereceiver 46 may be located at either a base station/Node-B or a userequipment. In current system implementations, AMC is typically only usedin the downlink. Accordingly, the preferred implementation oftransmission is for use in supporting AMC for the downlink. For othersystems using AMC in the uplink, transport block set transmission can beapplied to the uplink.

A transmitter 30 ₁ to 30 _(N) (30) transmits each TBS, TBS₁ to TBS_(N),over the air interface 36. The number of TBSs in the TTI depends on theTBS size and the modulation and coding set used for transmission. If themost robust modulation and coding set is used to ensure successfuldelivery, the TTI may only support one TBS. If a lesser robustmodulation and coding set is used to achieve higher effective datarates, multiple TBSs are sent in the TTI. Alternately, some TBSs may bedestined for a different receiver 46 ₁ to 46 _(K) (46), as shown in FIG.3B. Each TBS may also be sent to a different receiver 46 ₁ to 46 _(N)(46), as shown in FIG. 3C. This flexibility allows for greater radioresource utilization and efficiency.

A receiver 38 ₁ to 38 _(N) (38) receives each transmitted TBS. A H-ARQdecoder 42 ₁ to 42 _(N) (42) decodes each received TBS. Although in FIG.3 one transmitter 30, receiver 38 and H-ARQ decoder 42 is shown for eachTBS, one transmitter 30, receiver 38 and H-ARQ decoder 42 may handle allthe TBSs. For each TBS failing the quality test, a request forretransmission is made by the ARQ transmitter 40. An ARQ receiver 32receives the request and directs the appropriate TBS(s) to beretransmitted. The retransmitted TBS(s) are combined by the H-ARQdecoder(s) 42 and another quality test is performed. Once the TBS(s)passes the quality test, it is released for further processing. Since aTTI can contain multiple TBSs, preferably, a failure in one TBS does notnecessarily require retransmission of the entire TTI, which moreefficiently utilizes the radio resources.

An AMC controller 34 is also shown in FIGS. 3A, 3B and 3C. If thechannel conditions change, the AMC controller may initiate a change inthe modulation and code set used to transfer data. FIG. 4 is a flowdiagram illustrating such a change occurring in AMC betweenretransmissions. A TTI is transmitted having multiple TBSs andafterwards, a change in the modulation and coding set occurs, (step 50).To illustrate using FIG. 5, a TTI has three TBSs, TBS₁, TBS₂ and TBS₃applied at the least robust modulation and coding set to achieve themaximum data rate. The modulation and coding set in FIG. 5 changes sothat only one TBS can be transmitted subsequently. Referring back toFIG. 4, at least one of the TBSs is received with an unacceptablequality and a retransmission is required, (step 52). In the illustrationof FIG. 5, TBS₂ requires retransmission, as shown by a large “X”. TheTBS requiring retransmission is sent at the new modulation and codingset and combined with the prior TBS transmission, (step 54). As shown inFIG. 5, only TBS₂ is retransmitted and it is combined with the priorTBS₂ transmission. Although this example illustrates sending only oneTBS at the more robust modulation and coding set, it is also possiblethat two TBSs could be transmitted with the more robust modulation andcoding set within the TTI.

FIG. 6 is an illustration of multiple TBSs requiring retransmission.Three TBSs, TBS₁, TBS₂ and TBS₃, are transmitted in a TTI. A change inthe modulation and coding set occurs such that only one TBS can be sentat a time. All three TBSs are received with an unacceptable quality. Arequest for retransmission is sent for all three TBSs. Sequentially,each TBS is retransmitted, as shown by retransmission 1, retransmission2 and retransmission 3 in separate TTIs. The retransmitted TBSs arecombined with the prior transmissions. A similar procedure is used, iftwo TBSs are transmitted with the more robust modulation and coding setwithin the TTI.

As illustrated, multiple TBSs allow for maximum data rates andincremental redundancy. A TTI can be transmitted at the least robustmodulation and coding set achieving the maximum data rate and subsequentH-ARQ retransmission can be made at a more robust modulation and codingset ensuring greater probability for successful transmission. Byallowing incremental redundancy, radio resources can be used moreaggressively. A more aggressive (less robust) modulation and coding setcan be used to achieve higher data rates and radio resource efficiency,since transmission can be made using a more conservative (more robust)set to maintain QOS, if channel conditions degrade.

In a TDD/CDMA communication system, such as in the 3GPP system, twopreferred approaches for implementing multiple TBSs within a TTI useeither overlapping or non-overlapping time slots. In overlapping timeslots, the TBSs may overlap in time. As illustrated in FIG. 7, a firstTBS in a TTI uses the resource units having an “A” in them. A resourceunit is the use of one code in a time slot. A second TBS has the “B”resource units. As shown in FIG. 7, in the second time slot, both thefirst and second TBS are transmitted. Accordingly, the two TBSs'transmissions overlap in time.

In non-overlapping TBSs, each time slot only contains one TBS of a TTI.As illustrated in FIG. 8, a first TBS (“A”) is the only TBS in slots oneand two. The second TBS (“B”) is the only TBS in slots three and four.

In a FDD/CDMA communication system, such as in the third generationpartnership project proposed system, transmissions occur simultaneously.In a FDD/CDMA system, preferably each TBS is assigned a differentcode/frequency pair for transmission. In an OFDM system, preferably eachTBS is assigned a separate sub-carrier for transmission.

1. A user equipment configured to transmit data of a transmission timeinterval using adaptive modulation and coding and having a physicallayer hybrid automatic repeat request mechanism, the user equipmentcomprising: a transmitter configured to transmit data over an airinterface in a single transmission time interval with a first specifiedmodulation and coding scheme, the single transmission time intervalhaving a plurality of transport block sets and in response to receivinga repeat request for retransmission of at least one particular transportblock set, the transmitter configured to retransmit the at least one ofthe particular transport block sets; and an adaptive modulation andcoding controller configured to change the specified modulation andcoding scheme to a second specified modulation and coding scheme;whereby enabling a combining a particular transport block settransmitted at the first specified modulation and coding scheme with aretransmitted version of the particular transport block set transmittedat the second specified modulation and coding scheme.
 2. The userequipment of claim 1 using a time division duplex/code division multipleaccess air interface, wherein the transmitted transport block sets areseparated by time.
 3. The user equipment of claim 1 using a codedivision multiple access air interface wherein the transmitted transportblock sets are separated by codes.
 4. The user equipment of claim 1using a time division duplex/code division multiple access air interfacewherein the transmitted transport block sets are separated by time andcodes.
 5. The user equipment of claim 1 using an orthogonal frequencydivision multiple access air interface wherein the transport block setsare separated by sub-carriers.